This invention relates to cathode ray tubes for image displays and is particularly concerned with improving the performance of electron guns used in such tubes.
The cathode of an electron gun used in a cathode ray tube relies upon thermionic emission for the release of electrons which are formed into a beam by the gun. Resistive heating of the cathode to provide a proper emission temprature requires a finite amount of time after a cathode ray tube device such as a television display is turned on, during which time the display is inoperative. The amount of time required among various brands and models may vary 6 to 20 seconds for conventional indirectly heated cathodes. These short periods can seem relatively long and annoying to a viewer. To shorten this warm-up period, television receivers, for example, have been equipped with means for applying a partial current to keep the cathode warm and ready for quick start-up when the apparatus is turned on. However, the urgent need to conserve energy has ruled out this "keep alive"0 approach. As a result, development efforts are being directed to instant-on, directly heated cathodes; that is, filamentary-type cathodes that have no keep-alive current drain, but nevertheless reach operating temperatures within one or two seconds.
The potential benefits are numerous. In addition to the obvious advantage of no standby current drain, the instant-on cathode operates with lower power consumption. The instant turn-on of the television picture is attractive to the user. In addition, instant turn-on is not only desirable but necessary in certain display applications wherein imaged data must be quickly available, such as in an aircraft cockpit or on an automotive panel display.
Cathodes normally comprise two basic members--a supporting structure, and a deposited material which becomes electron-emissive when heated to a temperature in the range of 800-900 degrees centigrade. In conventional cathode structures, the electron-emissive material is heated to emission temperature by a separate, electrically isolated heating element. In cathodes of the quick-heating type, heating may be accomplished by routing current through the supporting structure itself, which offers the necessary electrical resistance to the passage of current to produce the proper operating temperature.
The following design criteria are preferred objectives for cathodes of the quick-heating type.
1. Low mass. Heat capacity must be small for quick heating, as any significant structural mass necessarily slows the heating of the cathode to the desired thermal equilibrium wherein a satisfactory flow of electrons will be emitted.
2. Physical stability. Any appreciable lateral displacement of the cathode structure upon heating can degrade electron gun performance. The displacement is manifested as a distortion in the beam spot projected by the gun which becomes other than round. Similarly, translation of the cathode in the longitudinal direction can affect the critical spacing between the cathode and the adjacent first grid; this can also result in beam spot distortion. The spacing must typically be held to within .+-. 0.0001 inch.
Some longitudinal translation is inevitable upon heating; however, it must be predictable. When the cathode is heated to its normal operating temperature, the excursion limit of any longitudinal translation must be repeatable to maintain the required cathode-G1 spacing.
3. Immunity to shock and vibration. This factor is especially important where cathode ray tubes are used in vehicles. Movement of the cathode in response to vibration can be very noticeable in the image display. Shock can result in permanent displacement of the cathode from its proper location, especially when it is in a heated and relatively plastic state and hence more vulnerable to shock.
4. Proper resistance. In addition to providing physical stability, practical values of heating current require a material having a high electrical resistance (greater than 100 microhm--cm).
5. Hot strength. Adequate hot strength at operating temperatures of 850 degrees centrigrade, for example, is a specific requirement for any filamentary-type cathode substrate alloy, apart from subtleties of trace elememt cathode activators.
6. Low power consumption. The desired average power consumption is in the range of 600 to 1000 milliwatts.
7. Long life potential. Operating characteristics must not change appreciably as a result of many thousands of on-off cylings, nor must there be any premature burn-out of the cathode.
8. Material compatability. The material of which the cathode structure is comprised, and the material of the electron-emissive deposit, must have nearly equivalent coefficients of expansion so that flaking and peeling of the deposit will not occur during repeated on-off cycling.
9. Environmental compatability. Cathodes must be capable of operation in the high-vacuum environment of the cathode ray tube without outgassing or releasing particulate matter.
10. Economical manufacture. Simplicity in structural design lowers manufacturing costs and helps ensure that all cathodes manufactured will operate in an identical manner.
U.S. Pat. No. 4,129,801 to Soeno et al, discloses a directly heated cathode comprising a cathode substrate body having two leg pieces extending in the same direction and including a flat part connected to one end of each leg piece. The body is prepared by shaping the flat metal plate of nickel or cobalt-based alloy, with a bonding layer applied having an uneven surface prepared by diffusion bonding by heating a powdered layer comprising powders of alloy or a mixture of nickel and cobalt formed on the flat part. A thermionic electron-emissivve layer is bonded to the flat part. An object of the invention is to provide a directly heated cathode free from thermal deformation.
Misumi et al, in U.S. Pat. No. 4,215,180, is addressed to oxide-coated cathodes containing as a base metal an alloy comprising at least one high-melting point metal. The object is to provide oxide-coated cathodes able to prevent an interfacial reaction between the alkaline earth oxides and the base metal plate which can result in peeling of the oxides from the base. The cathode structure upon which the invention is practiced is indicated as being a bare metal plate supported by two outwardly angled pillars resting on terminals connected to power source.
A direct heated cathode member for an electron tube is disclosed by Takanashi et al, in U.S. Pat. No. 3,541,382. A ribbon-shaped heater is stretched over an insulating substrate with its major surface very close to and parallel to that of the substrate. The ends of the ribbon-shaped heater are fitted to elastic metal strips so the heater is kept tensioned in parallel relation to the substrate, and a layer of thermion-emittilng material is deposited on the central part of the heater.
A three-piece, fast warm-up picture tube cathode system is disclosed by Buescher et al, in U.S. Pat. No. 3,881,124. A discrete cathode cap is supported by a separate structure which includes a plurality of relatively thin tabs attached to the cap. The number of such tabs is specified by Buescher et al, as being ". . . four in number (three tabs have been tried and are satisfactory) . . ." the use of such thin tabs is said to provide for poor heat conduction between the cap and the supporting structure necessary for fast warm-up. The third member is a heater filament located within the cap for heating the cap to the proper cathode emission temperature. Warm-up time is stated to be between five and six seconds.